Heart racing. Definitely of concern. Start by logging your symptoms and specifically circumstances when it happens, time of the day, completed/ about to start activities, mood, etc. Try to feel your pulse at the wrist and get a rate/min, have somebody else count and confirm also. Reduce as much as possible caffeine containing drinks. If pulse consistently over 120/min talk to your doc for a visit. Bring log.

Many possibilities. Could be due to many things: anxiety and stress; true heartrhythm abnormality; overactive thyroid among many others. Tests I would do would be a thorough history and physical examination, an Echocardiogram (ultrasound of the heart), routine EKG, blood work including a TSH to measure thyroid function, and a 24 EKG Holter monitor. That combination is more than likely to yield a diagnosis. .